Individuals with ASD have significant impairments in adaptive skills, particularly adaptive socialization skills. The present study examined the extent to which 20 items from the Vineland Adaptive Behavior Scales-Socialization Domain differentiated between ASD and developmentally delayed (DD) groups. Participants included 108 toddlers with ASD or DD under the age of 3 years. Nine of the 20 items significantly distinguished the groups. The ASD group demonstrated significantly weaker socialization skills, including deficits in basic social behaviors. The results support the notion that (a) socialization deficits in ASD impact foundational social skills typically emerging in the first year of life, (b) examination of specific social adaptive behaviors contribute to differential diagnosis, and (c) foundational social behaviors should be targeted for intervention.